Services that allow users to share videos on-line have become popular. In a typical service, a user uploads a video with tags that describe the video. The service maintains a searchable database of videos, and the user's video can appear in search results so that the video is findable and viewable by the world.
One issue that video-sharing services often have to address is that users may upload videos that the service does not want to show. One example of a video that the service might not want to show is a video that serves as an advertisement for a web site. Such a video may have image content, and also may have words referring a viewer to a particular web site—e.g., the video may display the Uniform Resource Locator (URL) of a web site superimposed across the images. In many cases, this type of video is used to advertise pornography, and the operator of the video sharing service may want to avoid being involved in the promotion of pornography. Thus, the operator of a video-sharing service may wish to treat videos that advertise web sites differently from other videos.